Comparison of extraction methods for the analysis of natural dyes in historical textiles by high-performance liquid chromatography

Abstract

Different methods for the extraction of Dactylopius coccus Costa, Rubia tinctorum L., Isatis tinctoria L., Reseda luteola L., Curcuma longa L. and Cotinus coggygria Scop. from wool fibres are investigated using high-performance liquid chromatography with diode array detector (HPLC-DAD). The efficiencies of five extraction methods which include the use of HCl (widely used extraction method), citric acid, oxalic acid, TFA and a combination of HCOOH and EDTA are compared on the basis of the (a) number, (b) relative quantities, measured as HPLC peak areas and (c) signal-to-noise ratios (S/N) of the compounds extracted from the wool substrates. Flavonoid glycosides and curcuminoids contained in R. luteola L. and C. longa L., respectively, according to liquid chromatography with mass spectrometry (LC-MS) identifications, are not detected after treating the fibres with HCl. All the other milder methods are successful in extracting these compounds. Experiments are performed using HPLC-DAD to compare the HPLC peak areas and the S/N of the following extracted compounds: indigotin, indirubin, curcumin, demethoxycurcumin, bisdemethoxycurcumin, fisetin, sulfuretin, luteolin, luteolin-7-O-glucoside, apigenin, carminic acid, alizarin, puruprin and rubiadin. It is shown that the TFA method provides overall the best results as it gives elevated extraction yields except for fisetin, luteolin, apigenin and luteolin-7-O-glucoside and highest S/N except for fisetin and luteolin-7-O-glucoside. It is noteworthy that treatment of the fibres with the typical HCl extraction method results overall in very low S/N. The TFA method is selected for further studies, as follows. First, it is applied on silk dyed samples and compared with the HCl method. The same relative differences of the TFA and HCl methods observed for the wool dyed samples are reported for the silk dyed samples too, except for rubiadin, luteolin and apigenin. Thus, in most cases, the nature of the substrate (wool or silk) appears to have negligible effects on the relative difference of the two extraction methods. Second, the selected TFA method is applied to treat wool and silk historical samples extracted from textiles of the Mamluk period, resulting in the identification of several colouring compounds. In all extraction methods mentioned above, DMSO is used to dissolve the dyes, after acid treatment.

Extracting natural dyes from wool—an evaluation of extraction methods

“…These originally green colours tend to become bluish when subjected to light due to the faster photodegradation of the flavonoid dyes. HCl-based methods are not suitable for the extraction of the yellow flavonoid dyes due to hydrolysis of the O-glycosidic bonds [16,21]. Being so, caution is needed when they are used to extract bluish samples since relevant information regarding the source of other natural dyes could be lost.…”

“…Surowiec et al [11] compared the two HCl procedures for the extraction of weld dye aglycones, concluding also that they result in very similar LC peak areas. However, the known hydrolysis of the flavonoid glycosides should be taken into consideration for the apparent good yields obtained with the HCl methods for the studied aglycones [21]. In fact, luteolin-7-O-glucoside (2, Table 1) is absent in chromatograms of the HCl methods, being only detected when milder extraction procedures were used.…”

“…The chemical structure and properties of the different dye chromophores suggest that probably no single procedure is adequate for the dye's extraction. Despite the obvious importance of the sample preparation methodology for the ultimate relevance of the analytical data collected, few systematic studies on the recovery rates of the different chromophores with the available extraction methods have been published [11,16,21]. One of the most commonly used extraction methods involves the use of hydrochloric acid (HCl) and methanol (MeOH) at high temperature [5-7, 11, 14, 16, 17].…”

“…When indigoids dyestuffs are present in the fibre, the use of DMF [27], hot pyridine [28], MeOH/DMF [11] or dimethylsulfoxide (DMSO) [21] for the residue solubilisation is recommended instead of MeOH or MeOH/H 2 O, since indigotin and indirubin are insoluble in these solvents [11,21].…”

“…Finally, in a recent work, Valianou et al [21] presented a systematic comparative study of five different dye extraction procedures based on hydrochloric, citric, oxalic and trifluoroacetic acids (TFA) and an HCOOH/H 2 EDTA mixture. All five extraction methods were tested on dyed wool with red and yellow mordant dyes (madder, cochineal, weld and young fustic), a yellow direct dye (tumeric) and a blue vat dye (woad) and also on historical textile samples.…”